Using a very simple single module project, the aggregated report is created by default and reports an incorrect number of violations, it just doubles the numbers. I checked out http://svn.apache.org/repos/asf/maven/plugins/tags/maven-checkstyle-plugin-2.8 and modified the integration test project {{checkstyle-report}} a little bit:
* I deleted the {{reportSets}} configuration from the {{pom.xml}}.
* I modified {{src/main/java/org/MyClass.java}} to actually have errors.

Both the {{checkstyle}} and {{checkstyle-aggregate}} report are generated and {{checkstyle-aggregate}} reports the same errors twice. I attach the patched project which includes a check in {{verify.groovy}} to assert that only one {{file}} entry is present in the created {{target/checkstyle-result.xml}}.

Not sure that the reported fix to the count problem MCHECKSTYLE-167 has been fixed in the checkstyle-aggregate report. If you have a look at http://maven.apache.org/plugins/maven-checkstyle-plugin/project-reports.html, you will see the report repetition AND that the first report has double the issues of the second report. In the checkstyle-aggregate report, each error is reported twice.